In general, a prism film is used to increase luminance of a backlight unit disposed at the rear surface of a liquid crystal display (LCD). Various attempts have been made to date to increase luminance of the backlight unit. High luminance of the backlight unit may be realized by appropriately controlling the light flow of the backlight unit. As such, the light flow may be controlled using a three-dimensional structure capable of suitably employing theories of interference, diffraction and polarization and photon theory, interpreted by the wave nature and particle nature of light. Further, by changing the physical properties of the material constituting such a three-dimensional structure, the light flow can be additionally controlled. Thereby, photons may be emitted toward a user's desired direction, and thus luminance may be increased in such a direction. A film manufactured to have a specific three-dimensional structure for increasing frontal luminance of a light source device is disclosed in U.S. Pat. Nos. 4,542,449 and 4,906,070 and Korean Patent Application No. 1986-0009868.
The film has a prism layer composed of a corrugated surface on one side and a smooth surface on the other side, in which the corrugated surface includes a plurality of uniform isosceles triangles linearly arrayed at about 45° relative to the smooth surface. Further, two films having such prism layers are aligned orthogonally to provide a luminance increase film (hereinafter, referred to as ‘prism film’).
As the important optical factor of the material constituting the prism layer of the prism film, there is a refractive index. As the refractive index is increased, performance of the prism film is improved. The prism film having a high refractive index is used to increase the efficiency of the backlight unit of the LCD, which is disclosed in Japanese Patent Laid-open Publication No. Hei. 5-127159. Additionally, the arrangement process is disclosed in Japanese Patent Laid-open Publication No. Hei. 5-323319.
The prism layer of the prism film is formed of a polymer resin polymerizable by a free radical, in particular a UV-curable resin. Typically, a UV-curable polymer having a high refractive index is exemplified by (meth)acrylate having at least one aromatic moiety, (meth)acrylate containing halogen except fluorine, or sulfur-containing (meth)acrylate. The polymer resins having high refractive indexes are appropriately mixed to manufacture a prism film, which is then applied to the backlight unit.
However, when the compound having an aromatic moiety, serving as the high-refraction resin, is exposed to UV light for a long period of time, a yellowing phenomenon may occur. That is, in the backlight unit, a cold cathode fluorescent lamp (CCFL) serving as a light source partially emits UV light. Thus, as the number of CCFLs is increased and an exposure time to the CCFL is lengthened, the amount of UV light accumulated in the high-refraction polymer resin is drastically increased, and therefore a chromophore may be produced by the aromatic moiety, undesirably generating yellowing. Such yellowing malfunctions to decrease performance of the prism film. Hence, the high-refraction composition for use in forming the prism layer of the prism film should have means for preventing yellowing due to UV light.
In addition to stability to UV light, the high-refraction composition for use in forming the prism layer of the prism film should have conditions capable of increasing surface strength while maintaining adhesion required to securely support the high-refraction polymer resin to the transparent substrate film. Moreover, if the viscosity of the high-refraction composition is high, such a composition is difficult to process. Accordingly, it is preferred that the composition be maintained in a liquid state at room temperature.
Presently, the development of a high-refraction composition satisfying all of the above requirements is urgently required.